In the SAGD industry, the produced water recovered from the SAGD production fluids and make-up water added to account for losses must be treated to remove various contaminants to meet boiler feed water specification. The contaminants include water hardness, silica, minerals, and residual oil/bitumen. If the water hardness, silica, and minerals are not removed from the water prior to steam generation, they will precipitate in the boiler causing reduced heat transfer, lower capacities, higher boiler tube temperatures, and ultimately failure of the boiler or extended boiler outages for cleaning and repairs. If the residual oil/bitumen is not removed from the water prior to steam generation, there will be foaming and fouling issues in the boiler drum and tubes, again leading to process upsets and shutdowns.
The majority of SAGD production facilities utilize hot or warm lime softening systems combined with Weak Acid Cation (WAC) ion exchange systems to treat produced and make-up water. However, this process does not produce a high quality boiler feed water and necessitates the use of Once Through Steam Generators (OTSG) which only partially boil the feed water (75-80%) to prevent scale deposition by maintaining solids in solution in the un-boiled water. This leads to energy inefficiency and excessive water disposal rates. OTSGs are custom built for the oil sands industry making them very costly compared to conventional boilers.
Recently some SAGD operators have adopted falling film evaporators that produce a high quality distilled water for boiler feed water, which has made it possible to shift to more conventional drum boilers. The combination of falling film evaporators and drum boilers results in much higher water recycle rates in a SAGD facility, which is becoming an increasingly critical environmental consideration.
The overall process in a single sump evaporator is simplified as follows. The feed water flows to the evaporator sump and is re-circulated through the tube side of a falling film heat exchanger. A small portion of the water will evaporate. A compressor increases the pressure and temperature of the vapour and sends it to the shell side of the falling film heat exchanger. Heat exchanged between the vapour and water acts to condense the vapour on the shell side to distilled water and evaporates a small portion of the water on the tube side. The distilled water is stored in a distillate tank and then pumped to the downstream consumers. FIG. 1 (PRIOR ART) illustrates a typical MVC evaporator configuration.
Evaporators have been used extensively in the mining and pulp and paper industries as means of concentrating solids into a brine or recovering water from waste streams. In these applications, the solid contaminants are generally soluble in water. However, the SAGD process can introduce contaminants that are not normally present or in different concentrations as a result of injecting steam into an underground reservoir that is recovered as hot water with the production fluids. Oil and water soluble solids present in the reservoir may cause variances in produced water quality at any given time, which can lead to operating problems in standard evaporator designs.
Operating companies are finding that there are many shortcomings with the current industry practice and evaporator system designs in SAGD facilities. The typical problems in evaporators in SAGD facilities include:
a) Hardness scaling
b) Silica deposit
c) Oil accumulation and foaming
d) Poor internal mist elimination performance
e) Compressor vibration and scaling caused by foaming
f) Large size preventing its use in mobile systems
Hardness (mineral ions such as Ca2+ or Mg2+) scaling and silica deposits are controlled by limiting the concentration, increasing the pH of the water, adding scale inhibitors such as calcium sulphate seed crystals as described for example in U.S. Pat. No. 7,681,643; or controlling the water recirculation through the falling film heat exchanger.
There is some attempt in conventional designs to deal with oil accumulation in the evaporator sump via a skim draw at some specific level in the sump. This scheme can only be effective if the sump level is precisely controlled at a level just above the skim draw nozzle. If the level is too high above the draw point, oil will accumulate and if falls below the draw nozzle, no liquid flow will be drawn off and again oil will accumulate. The oil accumulated in the evaporator sump causes excessive foaming. Antifoam chemicals are added to the feed water but the addition is not adequate to deal with excessive foam caused by oil accumulation in the sump.
Evaporator internals consist of a mist eliminating pad between the evaporator sump and the compressor inlet to remove fine water droplets, which is water washed with product distilled water on an intermittent basis. Without the water washing, salt from evaporated brine accumulates on the surface of the mist eliminating pad resulting in excessive pressure drop at the compressor suction. A stream of distilled water is sprayed on the top and the bottom of the mist eliminating pad to dissolve deposited scale and the contaminated water returned to the evaporator sump.
The present invention has made an improvement in handling accumulated oil in the evaporator, in prior filed U.S. provisional patent application 61/376,301 entitled A Water Evaporator for a Steam Assisted Gravity Drainage (SAGD) Central Processing Facility (CPF) System the contents of which is hereby incorporated by reference herein as if it was fully included herein with. This patent application also claims process controllers that will improve the handling of scaling and silica issues.
The implementation of existing technology involves a vertical exchanger bundle mounted on top of an evaporator sump. The sump provides both liquid inventory for the brine recirculation pumps and vapour space for liquid-vapour disengagement. The mist eliminators are typically installed in the vapour space in the annulus area around the falling film heat exchanger bundle, above which is outlet piping to the compressor suction. For example, DEMISTER® pads (DEMISTER® is a Registered trademark of Koch-Glitsch, LP) may be used as a specific mist eliminating pad. As a result, the evaporator is very tall relative to other SAGD equipment and has a vessel diameter which is significantly larger than the falling film exchanger. These dimensional features restrict the equipment capacity that can be easily modularized and transported. The high labour costs and low productivity typically associated with SAGD operations have driven owners to seek modular construction techniques to minimize site construction. This has created a need for a new design of evaporator for use in the modular SAGD technology/market development addressing all the above mentioned deficiencies.
There are also some operational and safety opportunities for adapting the old technology to the relatively new SAGD application. These opportunities include reducing the entrainment, reducing the foaming and scaling, improving the oil skimming, and reducing the maintenance and unexpected shutdowns.
Further and other aspects and objectives of the invention will become apparent to one skilled in the art from a review of the detailed description of the preferred embodiments of the invention illustrated and claimed herein.